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Subject ELECTRICAL SCIENCE Code BS-105
Community SnapED Community
Introduction to DC Generator
❖ The DC Generator is the device which converts the mechanical energy input into the
direct current output.
❖ The working principle of the DC generator is based on Faraday’s laws of electromagnetic
induction. When a conductor is located in a changing magnetic field, an electromotive
force(EMF) gets induced within the conductor.
❖ The necessary magnetic flux is produced by field winding( current carrying winding).
The direction of the induced e.m.f. can be obtained by using Fleming’s right hand rule.
Introduction to DC Motors
❖ The DC motor is the device which converts the direct current into the mechanical work.
❖ In the DC motor, the supply voltage E and current I is given to the electrical port or the
input port and we derive the mechanical output i.e. torque T and speed ω from the
mechanical port or output port.
It works on the principle of
Lorentz Law, which states that “the current-carrying conductor placed in a magnetic and
electric field experience a force”. The experienced force is called the Lorentz force. The
Flemming left-hand rule gives the direction of the force.
Construction of DC Machine
DC machine consists of four parts mainly
1. Field magnets
2. Armature
3. Commutator
4. Brush and brush gear.
A 4-Pole DC Motor is shown in the figure.
1. Field System
❖ The objective of the field system is to create a uniform magnetic field, within which the
armature rotates.
❖ Electromagnets are preferred in comparison with permanent magnets on account of its
greater magnetic effect & Field Regulation.
❖ Field magnet consists of four parts:
(i) Yoke or Frame (ii) Pole cores
(iii) Pole shoes and (iv) Magnetising coils.
Construction of DC Motors
1. Field System
❖ Cylindrical
yoke is usually used which acts as a frame of the machine and carries the magnetic flux
produced by the poles.
❖ Pole core is usually of circular section and is used to carry the coils of insulated wires
carrying the field current.
❖ The pole shoe acts as a support to the field coils and spreads out the flux over the armature
periphery more uniformly and also being of larger cross section reduces the reluctance of
the magnetic path.
❖ The objective of the magnetising or field coils is to provide the number of ampere-turns of
excitation required to give the proper flux through the armature to induce the desired
potential difference.
Construction of DC Motors
2. Armature
❖ It is a rotating part
of a dc machine and is built up in a cylindrical or drum shape. The purpose of armature is
to rotate the conductors in the uniform magnetic field.
❖ It consists of coils of insulated wires wound around an iron and so arranged that electric
currents are induced in these wires when the armature is rotated in a magnetic field.
❖ In addition, its most important
function is to provide a path of very low reluctance to magnetic flux. The armature core is
made from high permeability silicon-steel
❖ A small air gap exists between the pole pieces and the armature so that there will be no
rubbing in the machine. However this gap is kept as small as possible, since larger the air
gap greater is the mmf required to create the required flux.
Construction of DC Motors
3. Commutator
❖ The commutator is a form of rotating switch placed between the armature and the external
circuit and so arranged that it will reverse the connections to the external circuit at the
instant of each reversal of current in the armature coils. It is very important part of a dc
machine and serves the purposes of providing the electrical connections between the
rotating armature coils and the stationary external circuit.
❖ The commutator of DC motor is a cylindrical structure made up of copper segments
stacked together, but insulated from each other by mica.
4. Brushes
❖ The brushes of DC
motor are made with carbon or graphite structures, making sliding contact over the rotating
commutator. The brushes are used to relay the current from external circuit to the rotating
commutator form where it flows into the armature winding. So, the commutator and brush
unit of the DC motor is concerned with transmitting the power from the static electrical
circuit to the mechanically rotating region or the rotor.
Construction of DC Motors
5. Armature Winding
EMF Equation of DC Machine
Time taken to complete
one revolution is given as:
Therefore, the average induced e.m.f in one
conductor will be:
EMF Equation of DC Motors
Putting the value of (t) from Equation (2) in the equation (3) we will get
The number of conductors connected in
series in each parallel path = Z/A.
Therefore, the average induced e.m.f across each parallel path or the armature terminals is given
by the equation shown below:
Importance of Back EMF in DC Motors
❖ The back emf is developed in series with the applied voltage, but opposite in direction, i.e.,
the back emf opposes the current which causes it.
❖ A simple conventional circuit diagram of the machine working as a motor is shown in the
diagram below:
❖ Energy conversion in the DC motor is possible only because of the back emf. The
mechanical energy induced in the motor is the product of the back emf and the armature
current, i.e., EbIa.
❖ The back emf makes the DC motor self-regulating machine, i.e., the back emf develops
the armature current according to the need of the motor.
Types of DC Generators
Types of DC Motors
Permanent Magnet Motor
❖ It consists of an armature and
one or several permanent magnets encircling the armature. Field coils are usually not
required. However, some of these motors do have coils wound on the poles. If they exist,
these coils arc intended only for recharging the magnets in the event that they lose their
strength.
Separately-Excited DC Motors
❖ As the name
suggests, in case of a separately excited DC motor the supply is given separately to the
field and armature windings. The main distinguishing fact in these types of DC motor is
that, the armature current does not flow through the field windings, as the field winding is
energized from a separate external source of DC current as shown in the figure.
Types of DC Motors
Series Wound DC Motors
❖ As the name implies, the field
coils, consisting of few turns of thick wire, are connected in series with the armature, as
illustrated in Figure.
❖ The cross-sectional area of the wire used for field coils has to be fairly large to carry the
armature current, but owing to the higher current, the number of turns of wire in them need
not be large. In DC Series Motor,
Types of DC Motors
Shunt Wound DC Motors
❖ The word "shunt"
means "parallel". These motors are so named because they basically operate with the field
coils connected in parallel with the armature. The field winding consists of a large number
of turns of comparatively fine wire so as to provide large resistance.
❖ The field current is much less than the armature current, sometimes as low as 5%.
❖ The connection diagram is shown in Figure. The current supplied to the motor is divided
into two paths, one through the field winding and second through the armature.
Types of DC Motors
Compound Wound DC Motors
❖ A compound wound dc motor has
both shunt and series field coils. The shunt field is normally the stronger of the two (i.e.,
has more ampere-turns). Compound wound motors are of two types namely cumulative
compound wound and differential compound wound motors.
❖ Cumulative compound wound motor is one in which the field windings are connected in
such a way that the direction of flow of current is same in both of the field windings. In the
motor of this type the flux due to series field winding strengthens the field due to the shunt
field winding.
❖ Differential compound wound motor is one in which the field windings are connected in
such a way that the direction of flow of current is opposite to each other in the two field
windings. In this type of motor the flux due to series field winding weakens the field due to
shunt field winding.
Importance of Back EMF in DC Motors
❖ The back emf is developed in series with the applied voltage, but opposite in direction, i.e.,
the back emf opposes the current which causes it.
❖ A simple conventional circuit diagram of the machine working as a motor is shown in the
diagram below:
❖ Energy conversion in the DC motor is possible only because of the back emf. The
mechanical energy induced in the motor is the product of the back emf and the armature
current, i.e., EbIa.
❖ The back emf makes the DC motor self-regulating machine, i.e., the back emf develops
the armature current according to the need of the motor.
Types of DC Motors
Permanent Magnet Motor
❖ It consists of an armature and
one or several permanent magnets encircling the armature. Field coils are usually not
required. However, some of these motors do have coils wound on the poles. If they exist,
these coils arc intended only for recharging the magnets in the event that they lose their
strength.
Separately-Excited DC Motors
❖ As the name
suggests, in case of a separately excited DC motor the supply is given separately to the
field and armature windings. The main distinguishing fact in these types of DC motor is
that, the armature current does not flow through the field windings, as the field winding is
energized from a separate external source of DC current as shown in the figure.
Types of DC Motors
Series Wound DC Motors
❖ As the name implies, the field
coils, consisting of few turns of thick wire, are connected in series with the armature, as
illustrated in Figure.
❖ The cross-sectional area of the wire used for field coils has to be fairly large to carry the
armature current, but owing to the higher current, the number of turns of wire in them need
not be large. In DC Series Motor,
Types of DC Motors
Shunt Wound DC Motors
❖ The word "shunt"
means "parallel". These motors are so named because they basically operate with the field
coils connected in parallel with the armature. The field winding consists of a large number
of turns of comparatively fine wire so as to provide large resistance.
❖ The field current is much less than the armature current, sometimes as low as 5%.
❖ The connection diagram is shown in Figure. The current supplied to the motor is divided
into two paths, one through the field winding and second through the armature.
Types of DC Motors
Compound Wound DC Motors
❖ A compound wound dc motor has
both shunt and series field coils. The shunt field is normally the stronger of the two (i.e.,
has more ampere-turns). Compound wound motors are of two types namely cumulative
compound wound and differential compound wound motors.
❖ Cumulative compound wound motor is one in which the field windings are connected in
such a way that the direction of flow of current is same in both of the field windings. In the
motor of this type the flux due to series field winding strengthens the field due to the shunt
field winding.
❖ Differential compound wound motor is one in which the field windings are connected in
such a way that the direction of flow of current is opposite to each other in the two field
windings. In this type of motor the flux due to series field winding weakens the field due to
shunt field winding.
DC Motors
Separately Excited DC Motor
Self Excited DC Motor
Permanent Magnet Motor
Series Wound Motor
Shunt Wound Motor
Compound Wound Motor
NOTES
Subject ELECTRICAL SCIENCE Code BS-105
Community SnapED Community
What is an AC Motor?
The motor that converts the alternating current into mechanical power by using an
electromagnetic induction phenomenon is called an AC Motor. The stator and the rotor
are the two most important parts of the AC motors. The stator is the stationary part of
the motor. And the rotor is the rotating part of the motor. An AC motor may be single
phase or three phase.
● Working Principle of an AC Motor
The fundamental operation of an AC Motor depends on the principle of magnetism. The simple AC
Motor contains a coil of wire and two fixed magnets surrounding a shaft. When an electric (AC)
charge applies to the coil of wire, it becomes an electromagnet. This electromagnet generates a
magnetic field.
Inside the stator, there is a solid metal axle, a loop of wire, a coil, a squirrel cage made of metal bars
and some other freely rotating metal part that can conduct electricity. In an AC motor you send
power to the outer coils that make up the stator. The coils energized in pairs, in sequence, producing
a magnetic field that rotates around the outside of the motor.
Do you know, how does this rotating field make the motor move?
The rotor suspended inside the magnetic field. The magnetic field is constantly changing due to
rotation so, according to the law of electromagnetism, the magnetic field produces an electric current
inside the rotor.
If the conductor is a ring or a wire, the current flows around it in a loop. If the conductor is simply a
solid piece of metal, eddy currents flows around it instead. The induced current produces its own
magnetic field and according to another law of electromagnetism, the rotating magnetic field by
rotating as well. Simply described, When the magnets interact, the shaft and the coil of wires begin
to rotate,which operates the motor.
Types of an AC Motor
Mainly an AC motor classified into two types. They are the synchronous motor and the
induction motor.
1. Synchronous AC Motor
The motor that converts an AC electrical power into mechanical power and operates
only at the synchronous speed is the synchronous motor.
When supply is given to synchronous motor, a revolving field is set up. This field tries to
drag the rotor with it but because of rotor inertia it could not rotate. Hence,it does not
produce any starting torque. Thus, the synchronous motor is not a self-starting the
motor.
2. Induction or Asynchronous AC Motor
The machine which converts the AC electric power into mechanical power by using an
electromagnetic induction phenomenon in called an induction motor. The induction
motor classified into two types
● Single phase induction motor
● Three phase induction motors.
In an induction machine the armature winding serves as both the armature winding and
field winding. When the stator windings connect to an AC supply, the flux produces in
the air gap. The flux rotates at a fixed speed called synchronous speed. This rotating
flux induces voltages in the stator and rotor winding.
If the rotor circuit closed, the current flows through the rotor winding and react with the
rotating flux and it produces the torque. In the steady state, the rotor rotates at speed
very close to synchronous speed.
Applications of an AC Motor
AC Motors can be found in numerous home appliances and applications,
● Clocks
● Power tools
● Disk drives
● Washing Machines and other Home Appliances
● Audio turntables
● Fans
They can also be found in industrial applications,
● Pumps
● Blowers
● Conveyors
● Compressors
AC GENERATOR
What Is an AC Generator?
AC generator is a machine that converts mechanical energy into electrical energy. The
AC Generator’s input supply is mechanical energy supplied by steam turbines, gas
turbines and combustion engines. The output is alternating electrical power in the form
of alternating voltage and current.
AC generators work on the principle of Faraday’s law of electromagnetic induction,
which states that electromotive force – EMF or voltage – is generated in a
current-carrying conductor that cuts a uniform magnetic field. This can either be
achieved by rotating a conducting coil in a static magnetic field or rotating the magnetic
field containing the stationary conductor. The preferred arrangement is to keep the coil
stationary because it is easier to draw induced alternating current from a stationary
armature coil than from a rotating coil.
The generated EMF depends on the number of armature coil turns, magnetic field
strength, and the speed of the rotating field.
AC Generator Parts and Function
The various parts of an AC generator are:
● Field
● Armature
● Prime Mover
● Rotor
● Stator
● Slip Rings
The following are the functions of each of these components of an AC generator.
Field
The field consists of coils of conductors that receive a voltage from the source and
produce magnetic flux. The magnetic flux in the field cuts the armature to produce a
voltage. This voltage is the output voltage of the AC generator.
Armature
The part of an AC generator in which the voltage is produced is known as an armature.
This component primarily consists of coils of wire that are large enough to carry the
full-load current of the generator.
Prime Mover
The component used to drive the AC generator is known as a prime mover. The prime
mover could either be a diesel engine, a steam turbine, or a motor.
Rotor
The rotating component of the generator is known as a rotor. The generator’s prime
mover drive the rotor.
Stator
The stator is the stationary part of an AC generator. The stator core comprises a
lamination of steel alloys or magnetic iron to minimise the eddy current losses.
Slip Rings
Slip rings are electrical connections used to transfer power to and fro from the rotor of
an AC generator. They are typically designed to conduct the flow of current from a
stationary device to a rotating one.
Working of an AC Generator
When the armature rotates between the poles of the magnet upon an axis perpendicular
to the magnetic field, the flux linkage of the armature changes continuously. As a result,
an electric current flows through the galvanometer and the slip rings and brushes. The
galvanometer swings between positive and negative values. This indicates that there is
an alternating current flowing through the galvanometer. The direction of the induced
current can be identified using Fleming’s Right-Hand Rule.
